1. Field of the Invention
Aspects of the present invention relate to a fixing device and an image forming apparatus. More particularly, aspects of the present invention relate to a fixing device that transforms an electrostatic latent image formed on a photoconductive medium into a toner image, transfers the toner image onto a recording medium, and fixes the transferred image on the recording medium by heat and pressure, and an image forming apparatus using the same.
2. Description of the Related Art
A fixing device is an essential part of electrophotographic image forming apparatuses such as a copiers, facsimile machines, and laser printers, since it creates a fixed image on a recording medium and determines the final image quality.
FIG. 1 illustrates the structure of a conventional fixing device as disclosed in Japanese Patent Publication No. 2003-280439.
Referring to FIG. 1, a conventional fixing device 20 comprises a fixing roller 21 coated with an elastic layer on the surface as a belt base material, a heating roller 23 having therein a heating source 22 such as a halogen lamp, and an auxiliary roller 24 assisting separation. Also, a fixing belt 25 winds around the fixing rollers 21, the heating roller 23, and the auxiliary roller 24 in a triangle-shaped path. Furthermore, a pressing roller 26 by which a pressure welding is carried out against the fixing roller 21 is formed in the fixing device 20, and a heating source 27 is built also into the pressing roller 26. The location which the pressure welding by the pressing roller 26 is carried out against the fixing roller 21 is referred to as a fixing nip.
A recording medium P which supports a non-fixed toner image is conveyed to the fixing device 20 by moving in the direction of the arrow-head A. The recording medium P is supported by a guide member 28 as it moves toward the fixing device. In addition, a press member 40 presses the fixing belt 25 between the fixing roller 21 of the fixing belt 25 and the auxiliary roller 24.
In the above-structured fixing device 20, when the leading end of the recording medium P has passed through the fixing nip, the leading edge of the recording medium is conveyed to a nip between the fixing belt 25 and the pressing member 40. Since the fixing belt 25 is bent as the recording medium P passes through the nip, the leading end of the recording medium P is separated from the fixing belt 25 and moved along a peripheral surface of the pressing member 40, and the separability of the recording medium P with respect to the fixing belt 25 is thus improved. A guide member 30 guides the movement of the recording medium P after the recording medium P passes the pressing member 40. An elastic member 33 elastically supports the heating roller 23.
FIG. 2 is an enlarged view showing a structure of a conventional image fixing device.
Referring to FIG. 2, an image fixing device 50 comprises a heating roller 51 having therein a heating source 51a, for example, a halogen lamp, and a pressing roller 53 forming a contact nip N by contact with the heating roller 51.
The heating roller 51 comprises a core pipe 51b and an elastic rubber layer 51c coated on an outer surface of the core pipe 51b. 
The pressing roller 53 comprises a metal core pipe 53b and an elastic rubber layer 53c coated on an outer surface of the metal core pipe 53b. 
In the above-structured fixing device, before the image fixing process, the fixing nip N of the fixing device needs to be heated up to a predetermined fixing temperature and kept at the fixing temperature.
According to the related art, the core pipe 51b of the heating roller 51 is heated by calorific operation of the heating source 51a, and the heat is conducted up to the elastic rubber layer 51c, thereby heating and keeping the elastic rubber layer 51c at the predetermined fixing temperature. As a non-fixed toner image formed on the recording medium P passes through the fixing nip N, the toner image is heated and pressurized, thereby being fixed on the recording medium P.
When color-electrophotographic image forming apparatuses are used in high-speed operations, it is necessary to increase the outer diameters of the rollers 51 and 53 or the thickness of the elastic rubber layers 51c and 53c to expand the width of the fixing nip N. Otherwise, in a high speed operation, as the printing speed increases, the fixing quality deteriorates due to a decreased staying time of a recording medium P at the fixing nip N.
However, increase in the diameters of the rollers 51 and 53 increases the whole size of the apparatus, increases the warming-up time, and increases the manufacturing cost.
Moreover, when the thickness of the elastic rubber layers 51c and 53c is increased, the warming-up time is increased. Furthermore, the inner temperature of the core pipe 51b needs to be much higher so as to maintain the fixing temperature of the thickened elastic rubber layer 51c. The higher temperature causes the surface of the core pipe 51b, the contacting part between the core pipe 51b and the elastic rubber layer 51c, or the elastic rubber layer 51c to deteriorate. Also, durability of these parts is decreased.
In order to ensure the fixing quality in a high-speed image forming apparatus, the pressure applied to the fixing nip N may be increased. However, too much pressure at the fixing nip N would deform the elastic rubber layers 51c and 53c, thereby reducing their durability. Furthermore, paper jams would occur more frequently because the recording medium P cannot move smoothly. Moreover, as the driving torque increases, the apparatus may be damaged. Moreover, when the pressing member has a relatively small length, the pressing member may be deformed or bent by the high pressure, thereby considerably reducing the width of the fixing nip N at the center portion compared to the opposite end portions. In this case, evenness of the fixing nip N through the whole length cannot be obtained.
FIG. 3 is a graph illustrating the distribution of pressure at the fixing nip N in the conventional fixing device.
Referring to FIG. 3, the distribution of pressure at the fixing nip N is symmetrical between an inlet I and an outlet O. Therefore, when the recording medium P passes through the outlet O, a printed side of the recording medium P may adhere to the surface of the heating roller 51 and roll up. This phenomenon is called ‘wrap jam’.